I'm speaking of the prime universe being overwritten from the point of view of the characters Christopher. I'm simply pointing out that there's nothing in the phasing used in the movie that says definitively one way or the other. No ret-con would be needed if, for whatever reason, TPTB decided that the prime universe was overwritten as it was in City on the Edge of Forever or First Contact. In those cases it was restored but we aren't sure one way or the other of that is the case this time. As we'll probably never see the canon version of the prime universe again it doesn't matter if it has been overwritten or not. You choose to say that it wasn't. I prefer to think that it's possible that it was. Not that it was, just that it's possible.